1. Field of the Invention
This invention relates to improvements in operational amplifiers, and more particularly to improvements in operational amplifiers that have wide operating voltage ranges approaching rail-to-rail swings, and still more particularly to techniques in reducing the size and complexity of operational amplifiers.
2. Background Information
Recently, increased interest has been directed towards realizing low distortion class AB operational amplifiers that have a wide range of input and output voltages, approaching the upper and lower supply voltages rails. It is desirable, of course that the amplifier be capable of being realized in a monolithic integrated circuit structure of bipolar or MOS transistor devices. In the past, however, only large-size, often three-stage structures have been proposed to achieve the desired performance, resulting in a large cost in silicon area, particularly for circuits having to work under low supply constraints.
For example, Johan H. Huijsing et al., in "Low-Voltage Operational Amplifier with Rail-to-Rail Input and Output Ranges", IEEE Journal of Solid-State Circuits, Vol. SC-20, No. 6, December, 1985, discloses an operational amplifier that can operate with a supply voltage as low as 1.5 volts. The output voltage can reach the supply rail within 150 mV. The circuit has an input stage that provides rail-to-rail common-mode voltage range and an output stage with rail-to-rail output-voltage swing and accurate class AB control. The circuit, however, requires an intermediate stage to allow the input and output stages to be connected together to produce sufficient voltage gain to enable the circuit to be used as a general purpose operational amplifier. The provision of the third stage not only increases the size and complexity of the circuit, it reduces the circuit bandwidth and speed.
Jeroen Fonderie et al., in "1-V Operational Amplifier with Rail-to-Rail Input and Output Ranges", IEEE Journal of Solid-State Circuits, Vol. 24, No. 6, December, 1989, shows a bipolar operational amplifier with rail-to-rail input and output ranges for low supply voltages. This circuit again requires a large number of components and an intermediate stage between the input and output stages. Being a three-stage design, its composition is relatively complicated, and has a bandwidth limited to approximately 450 kHz.